The telecommunication industry is turning towards optical networks to provide higher capacity (bandwidth) of data transfer for new applications. Consequently, the industry is continually developing technology to increase the bandwidth of the optical networks, which includes the technology of wavelength division multiplexing (WDM). A number of optical signals can be carried simultaneously on the optical fiber by modulating each signal on a different wavelength of light. The light propagating through a single optical fiber therefore includes a plurality of wavelength bands, referred to as channels, wherein each channel or wavelength band is centered at a respective wavelength. The wavelength bands are sufficiently separated so that they do not overlap. Typically, networks that carry approximately eight channels or more are referred to as dense wavelength division multiplexing (DWDM) systems, such systems can carry greater than 200 channels.
The technology of optical networks is heading toward a more dynamic, flexible, and intelligent networking architecture to improve service delivery time. A key element of the emerging optical network is a tunable channel optical filter for selectively filtering a channel from the DWDM light propagating through the optical network. The tunable channel filter enables a channel to be dynamically switched, routed, monitored and conditioned.
It is known in the art of optical networks that Bragg gratings written in an optical fiber may be used to act as a tunable filter, as is described in U.S. Pat. No. 5,815,299, entitled “Method and System for Equalizing Respective Power Levels of Channels of a Received Optical Frequency Division Multiplexed Signal” to Bayart et al.; U.S. Pat. No. 5,995,255, entitled “Concatenated Fiber Grating Optical Monitor” to Giles, and U.S. Pat. No. 5,579,143, entitled “Optical System With Tunable In-Fiber Gratings” to Huber.
The tunable filter described in the aforementioned U.S. Pat. Nos. 5,815,299 and 5,995,155 use a single fiber Bragg grating to filter a selected optical channel or a limited number of channels from broadband light tapped off an optical network. Consequently, the optical system requires a number of tunable filters to monitor or condition each optical channel. In some instances, the number of tunable filters can equal the number of optical channels.
It is also known in the art to add a light signal to a fiber optic transmission line using a device comprising a pair of substantially identical Bragg-Evanescent-Couplers (BEC) devices that are cascaded in series, whereby both BECs reflect light having wavelengths in the region of λn, as described in U.S. Pat. No. 5,459,801 to Snitzer, entitled “Coupler Used To Fabricate Add-Drop Devices, Dispersion Compensators, Amplifiers, Oscillators, Superluminescent Devices, and Communications Systems”.
It is further known, as described in U.S. Pat. No. 5,841,918 to Li, entitled “Wavelength and Bandwidth Tunable Optical System” of a system for modifying an input optical signal by reducing its bandwidth and/or modifying its central wavelength. The system includes at least two optical filters optically coupled to a circulator. The first filter produces a predetermined reflected band of the input optical signal. The second filter produces a predetermined transmission band as part of the reflected band. One or both of the filters are tunable to modify the wavelength and/or bandwidth of the optical filter.